1. Field of the Invention
The present invention relates to a technique for a light emitting device using a light emitting element, and more specifically to a technique for a light emitting device capable of supplying a predetermined charge to a light emitting element.
2. Description of the Related Art
In recent years, the development of a display device for displaying an image has been progressed. As the display device, a liquid crystal display device for displaying an image using a liquid crystal element has been widely used for a display screen of a mobile telephone by taking advantages of a high image quality, a thin type, a light weight, and the like.
On the other hand, in recent years, the development of a light emitting device using a light emitting element has been also progressed. The light emitting device has features such as a high response speed, superior moving picture display, and a wide viewing characteristic in addition to an advantage of an existing liquid crystal display device. Thus, it has been noted as a next-generation compact mobile flat panel display capable of using moving picture contents.
The light emitting element is made of a broad material such as an organic material, an inorganic material, a thin film material, a bulk material, or a dispersion material. Of them, as a typical light emitting element, there is an organic light emitting diode (OLED) mainly made of an organic material. The light emitting element has a structure in which an anode, a cathode, and a light emitting layer sandwiched between the anode and the cathode are provided. The light emitting layer is made of one or plural materials selected from the above-mentioned materials. Note that the amount of current flowing between both electrodes of the light emitting element and light emission intensity have a directly proportional relationship.
In many cases, a plurality of pixels each having a light emitting element and at least two transistors are provided in the light emitting device. In each of the pixels, a transistor connected in series with the light emitting element (hereinafter indicated as a driver transistor) has a function for controlling light emission of the light emitting element. When a gate-source voltage (hereinafter indicated as VGS) of a driver transistor and a source-drain voltage (hereinafter indicated as VDS) thereof are changed as appropriate, the driver transistor can be operated in mainly a linear region or in mainly a saturation region.
When the driver transistor is operated in mainly the linear region (|VGS−Vth|>|VDS|), the amount of current flowing between both electrodes of the light emitting element is changed according to both values of |VGS| and |VDS|. Note that a drive method of operating the driver transistor in mainly the linear region is called constant voltage drive. FIG. 7B is a schematic view of a pixel to which the constant voltage drive is applied. In the constant voltage drive, the driver transistor is used as a switch, and a power source line and the light emitting element are shorted if necessary, thereby flowing a current into the light emitting element.
On the other hand, when the driver transistor is operated in mainly the saturation region (|VGS−Vth|<|VDS|), the amount of current flowing between both electrodes of the light emitting element is greatly dependent on a change in |VGS | of the driver transistor but not dependent on a change in |VDS|. Note that a drive method of operating the driver transistor in mainly the saturation region is called constant current drive. FIG. 7A is a schematic view of a pixel to which the constant current drive is applied. In the constant current drive, a gate electrode of the driver transistor is controlled to flow the necessary amount of current into the light emitting element. In other words, the driver transistor is used as a voltage control current source and the driver transistor is set such that a constant current flows between a power source line and the light emitting element.
There is a light emitting device using a pixel including three transistors, a capacitor element, and a light emitting element and employing a time gradation method in addition to the above-mentioned constant voltage drive (see Patent References 1 and 2).
[Patent Reference 1] JP 2001-343933 A
[Patent Reference 2] JP 2001-5426 A
The light emitting device to which the above-mentioned constant voltage drive is applied is influenced by deterioration of the light emitting element resulting from a change over time. More specifically, when a voltage-current characteristic of the light emitting element is deteriorated due to a change over time, the amount of current flowing between both electrodes of the light emitting element becomes smaller, so that a desirable light emission intensity cannot be obtained.
On the other hand, according to the light emitting device to which the constant current drive is applied, a set current is supplied between both electrodes of the light emitting element. Thus, the influence of deterioration of the light emitting element resulting from a change over time can be suppressed. However, when characteristics such as mobility and a threshold value of the driver transistor are varied, there is caused a variation in the amount of current supplied to the light emitting element. In other words, a display screen is directly influenced by a variation in characteristic of the driver transistor. Thus, unevenness of the entire display screen is caused.
Also, in FIGS. 7A and 7B, in many cases, an n-channel transistor has been used as a switching TFT (thin film transistor), and a p-channel transistor has been used as the driver transistor from relation of source ground. Therefore, a complicated process in which transistors having different conductivity types are manufactured on an insulating surface or a semiconductor substrate causes a reduction in yield and a rise in cost.